Preparation of flexible TiO₂ photoelectrodes for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO₂ photoelectrodes on indium tin oxide (ITO) coated polymer substrates have drawn great attention due to its lightweight, flexibility and advantages in commercial applications. However, the thermal instability of polymer substrates limits the process temperature to below 150 °C. In order to assure high and firm interparticle connection between TiO₂ nanocrystals (TiO₂-NC) and polymer substrates, the post-treatment of flexible TiO₂ photoelectrodes (F-TiO₂-PE) by mechanical compression was employed. In this work, Degussa P25 TiO₂-NC was mixed with tert-butyl alcohol and DI-water to form TiO₂ paste. F-TiO₂-PE was then prepared by coating the TiO2 paste onto ITO coated polyethylene terephthalate (PET) substrate using doctor blade followed by low temperature sintering at 120 °C for 2 hours. To study the effect of mechanical compression, we applied 50 and 100 kg/cm2 pressure on TiO₂/PET to complete the fabrication of F-TiO₂-PE. The surface morphology of F-TiO₂-PE was characterized using scanning electron microscopy. The resultant F-TiO₂-PE sample exhibited a smooth, crack-free structure indicating the great improvement in the interparticle connection of TiO₂-NC. Increase of compression pressure could lead to the increase of DSSC photoconversion efficiency. The best photoconversion efficiency of 4.19 % (open circuit voltage (Voc) = 0.79 V, short-circuit photocurrent density (Jsc) = 7.75 mA/cm2, fill factor (FF) = 0.68) was obtained for the F-TiO₂-PE device, which showed great enhancement compared with the F-TiO₂-PE cell without compression treatment. The effect of compression in DSSC performance was vindicated by the electrochemical impedance spectroscopy measurement.